Introduction: How to Make a Trailer Mounted Bike Sound System

A few years ago I started going to the monthly London Critical Mass bike rides.  Quite often other riders would bring along sound systems, ranging from a couple of speakers on a rear rack to large trailer mounted systems that needed a tandem to tow them!  After one particularly memorable Mass where I spent most of the evening behind one sound system playing really cool stuff, I decided to build my own.  My first effort (christened SoundWave by my teenage son) was OK, but I felt I could do better.  This instructable documents the construction of my second system, christened BeastBox (OK, he's a Transformers fan).

First off, I must say that the design of BeastBox owes a lot to a sound system called Son of Pedals, the designer of which put up a very helpful web page .  Without that one page of notes I guess I would have got there in the end, but it would have been a lot harder!  I'll refer to Son of Pedals and that one page of notes throughout this Instructable and to save time I'll just use SoP.

BeastBox is a trailer mounted sound system that can just about be towed by a reasonably fit person on a mountain bike.  It weighs 54kg and will play seriously loud music with proper gut-thumping bass for 3-4 hours.  I borrowed a sound meter from work and clocked it at 120dB(A) at 1m, not bad for something that runs off torch batteries :-D

Someone shot some video of the March 2011 Critical Mass ride and caught me a few times along the way.  The guy kindly let me edit the footage and I have uploaded a new video showing just the clips of BeastBox to give an idea of the sound level.  The part at 1:30 under the bridge was very loud - you can hear it is overloading the camcorder's microphone!

I have used it on a number of Critical Mass rides and a few other cycle events in and around London over the last 6 months.  I nearly always get a few people asking how it works, what it runs off, where I got the plans etc. and a friend of mine suggested I write an instructable for it.  This is my first Instructable so I'll welcome any comments, good or bad!

Step 1: It's All About Efficiency

A sound system loud enough to dance to needs more power than you can generate by pedalling so you have to rely on batteries to carry your energy around with you.  Batteries are HEAVY so you need to be as efficient as possible to get the maximum volume and play time from the limited stored charge you are carrying.  The bass driver soaks up the majority of the power because to create decent bass you basically have to shove large volumes of air back and forth so the bass driver and its cabinet is the most important factor in the design.

Like SoP, Beastbox uses a 6th order bandpass box for the woofer.  This is a two chamber enclosure with the driver mounted in the dividing partition.  Each chamber is tuned to a different frequency so that the effect is to get a big bass boost at those two frequencies and some boost over the ones in between.  The frequency response comes out a bit lumpy but you get a 5-10dB boost which makes a big difference.

The other important factor is the amplifier(s).  The obvious choice of amplifier is some sort of car audio unit.  My first sound system used a second hand car amplifier and wow, did it get HOT!  All of that heat is wasted energy so an efficient amplifier was a priority.  Fortunately you can now get Class D amplifiers which are much more efficient than older units.  Until recently you could only get Class D bass amplifiers but there are now full range Class D amps which cover the complete audio range.  Beastbox uses two Class D amps, one for the bass and one for the midrange and tweeter. 

Step 2: Trailer

If you are thinking of building a system like this, the first thing to do is decide on a trailer.  Most often, I guess you may already have a trailer or know where to get one from and the size of the trailer's load bed will govern the size of the main enclosure.  I used a Y-large trailer from Carry Freedom .  I already had this trailer and they are an excellent design with removable wheels for storage and a clever flexible connection to your bike so that you hardly know it's there (except when going up hills!).  Actually the "hardly know it's there" part can be a problem if you try to go round corners too fast!  The load bed on a Y-large is 50x70cm but you can carry larger loads than that, they just overhang a bit.  The main enclosure of Beastbox is 60cm front to back and 65cm wide.

Other candidates are frames from child trailers (lots of them on eBay) or even a DIY trailer.

Step 3: Main Enclosure and Bass Driver

A 6th order bandpass box has two chambers with the driver mounted in the dividing partition between the chambers.  Each chamber must be "tuned" to a specific resonant frequency.  This is done by a combination of setting the chamber volume and the length and area of a tuned port that protrudes into the chamber.  SoP uses one rearward firing port and two smaller downward firing ports.  For Beastbox, I changed the arrangement somewhat so that there are two rearward firing ports which have the same area so that the back end of the box looks symmetrical.  Also I figured that firing the sound out horizontally would be better than at the ground.  The ports can be any shape but if you are building them out of wood a rectangular shape is most convenient.  If you want to use circular ports you can buy ready made port tubes (search eBay) which you can cut to the required length.  I couldn't find any big enough for Beastbox so I went for rectangular.

Before starting on the box design you need to choose a bass driver because the chamber volumes have to be tuned to the driver you intend to use.  I used a Fane Sovereign 15-400 15 inch 400W driver.  The 400W is probably a bit of overkill but I could get a more or less unused one cheaply.  A better choice would have been a driver that uses a Neodymium magnet like on SoP because these are a lot lighter but they are harder to find in large sizes.

To design the 6th order bandpass box for Beastbox I used a program called WinISD .  I don't think this has been updated for a while and I couldn't get it to run under Windows 7 but it works fine in XP mode.  If your driver doesn't already exist in WinISD's database you have to define the driver by inputting its Thiele-Small parameters.  These will always be available on the manufacturer's website for any serious bass driver.  Once you have the driver defined it's basically a process of trial and error adjusting the box and port dimensions to get the chamber volumes correct while keeping the overall size within the range that will fit on your chosen trailer.

One thing to bear in mind is that the chamber volumes are net volumes - i.e. you have to subtract the volume of the batteries, amplifiers and anything else that takes up space inside the chambers (including the driver and the volume of air enclosed by the ports).  I found that a simple spreadsheet was helpful to keep track of the weights and volumes of the components and also to work out the chamber volumes for given external dimensions and port areas and lengths.  The front chamber is 70 litres and is tuned to 60Hz and the rear chamber is 35 litres and tuned to 130Hz.  The design gives a nearly 6dB boost at 77 Hz and over 8db at 150Hz.

Once you have the volumes and dimensions for the box and ports you can roughly work out where all the other components are going to fit.  I used Google Sketchup to design the box and it was very helpful.  The Sketchup files are attached.

The box is mostly constructed from 12mm birch plywood screwed and glued together.  The port tubes are made from 6mm birch plywood.  MDF would be another choice but I think it is more dense than plywood so it would make for a heavier box.  Whatever you use, the main points to note are that the box must be rigid and there mustn't be any gaps.  Rigidity is important because you want the panel resonances to be much higher than the bass frequencies that the box is producing.  Basically if you rap on the side of the box it wants to sound "tight" rather than boomy.  Gap-free construction is important because the bass driver generates large pressure fluctuations inside the box and if there are holes you will get whistling noises!

I cannot over emphasise how much it helps if you can get the panels accurately cut to size.  I have a friend in a woodworking shop who cut mine for me and it makes assembling them much more straightforward if they are all the correct size with clean, square edges.  I used 3.5 x 30 wood screws (No 6 x 1 1/4" for any Imperial fans out there) at 100mm intervals along each joint.  Pre-drill pilot holes for the screws so that you don't split the panels.  Make sure you use plenty of wood glue (PVA) on the joint, you can always wipe off the excess with a damp cloth.  A couple of long clamps are very useful to hold the panels in place while drilling the edges and waiting for the glue to dry.

The front chamber of the enclosure has a removable cover which is held in place by the midrange/tweeter mount and some tie downs.  This allows easy access to fiddle with the electronics and remove the batteries.  The cover is sealed on to the chamber by a strip of neoprene draught excluder tape which runs all around the edge.

Step 4: Mid-range and Tweeter

The mid range and tweeter section is much easier - once you have some suitable drivers! SoP uses a large horn speaker from P-Audio for the mid-range but these are very difficult to find. I understand P-Audio will make them but only if someone orders a large number. I searched eBay and the web for days before finally deciding on a pair of P108 40W 8 ohm horns from Henry's Electronics. Similar horns are also widely available on eBay. They are connected in parallel so their effective impedance is 4 Ohms.

The tweeter is a second-hand Fane CD140H. Not much to say about that other than it tweets well.

The horns and the tweeter are mounted in a simple wooden frame that sits on top of the main enclosure, held in place by the straps that hold the cover on. Making the horns and tweeter removable is handy if you need to transport or store the system.

Step 5: Mid-range Update

On one of its early outings I realised that one of the midrange horns had stopped working.  I replaced it with a new one.  Then, while I was writing this Instructable, my teenage son borrowed Beastbox for a friend's birthday party and it came back with another blown horn. When I dismantled the horn the coil had more or less melted so I don't know what volume they were running it at but I doubt it was very popular with the neighbours!  I think the problem is that because the horns are connected in parallel, if they have slightly different impedances one is going to get a lot more power that the other. I did some more searching on the web and I found a reasonable looking alternative driver and horn . Unfortunately it is a 16 ohm driver which I thought might be a problem but I got one to test and it actually works fine. I had to turn up the midrange crossover channel but it sounds quite a bit better than the two original horns.  It's more expensive but on the other hand it's cheaper than buying new horns every time I take it out!

I had to build a new midrange/tweeter mount to hold the new horn and I based the design on the one SoP uses.  I again modelled the mount in Sketchup and built it out of screwed and glued 12mm ply the same as used for the main enclosure.

Step 6: Mid-Range Update Dec 2012

It's been a while since I posted any updates to this instructable. Since I wrote this I have used beast box on several critical Mass rides and a few other organised bike rides as well. It's also been in demand from my 18-year-old son and all of his friends for 18th birthday parties! During that time I've had a few ongoing problems with the tweeters. I think this is probably because the current crossover cannot filter out enough of the mid-range frequencies to stop them getting to the tweeter and overloading it.

In an effort to fix these problems once and for all I did some more searching on the web and finally found a mid range horn which could actually cover the whole frequency range from the bass right up to the highest frequencies. This new mid-range driver is the P-Audio BM2-D750 which can handle 100W RMS. I used a P-Audio PH-2380 horn with the driver.  Together they were £130 from Blue Aran. I had to build yet another mid-range mount but realised that I didn't actually need a separate cover for the main unit but the base of the mid-range could form the cover for the main unit. The mid-range mount is made from 12 mm marine ply screwed and glued like the rest of the construction. Because there is now only one mid-range driver instead of a mid range and a tweeter I could replace the current crossover with a single channel one although I haven't done that yet.

Step 7: Batteries

The batteries are probably the heaviest single component of the system.  Since I was going to be using car audio amplifiers the obvious choice was a car or leisure type lead-acid battery.  Unfortunately lead is really heavy!  What you are looking for in an ideal battery is energy density - the amount of energy the battery can store per kg.  The best choice would be lithium ion or lithium polymer batteries but these have to be charged and discharged carefully or there is a serious risk of fire.  I didn't much fancy pedalling along towing a flaming wooden box so I decided against them.  The next best choice is nickel-metal-hydride (NiMH) batteries.  Individual NiMH cells have a nominal voltage of 1.2V so using 12 of them in series gives a pack voltage of 14.4V.  Car audio equipment is actually designed to run off around 14V because a car electrical system sits at around that voltage when the alternator is running so a 12 cell pack is ideal.

Beastbox uses 36 11,000mAh D size NiMH cells arranged in 3 packs of 12 cells.  Each pack was constructed from 3 x 4-cell battery holders connected together so that the 12 cells are in series.  The battery holders are connected together with heavy gauge wire so that the cells are all in series then the whole pack is taped together with gaffer tape.  I fitted a 10A miniature circuit breaker to each pack to protect against short circuits.  Each pack has a Deans-type connector so that it can be disconnected for removal and charging.  Remember to put the female Deans connector on the battery pack to help minimise the risk of short circuits.

The 3 packs are connected in parallel to give a total capacity of 33Ah at 14.4V, or 475Wh of charge.  This might not sound like a lot but it is enough for at least 3-4 hours running time.  Connecting NiMH packs in parallel needs a bit of care because the packs will inevitably have slightly different terminal voltages.  If you just connect them directly together the pack with the highest voltage will try to charge the others.  Unfortunately NiMH cells have (I think) a negative voltage/temperature characteristic so that as the charging cells heat up their voltage falls and they charge up more.  They then take more current from the "donor" pack etc etc. You can see where this is going!  The solution is to connect the packs in parallel through Schottky diodes which act like non-return valves, allowing current to flow out of each pack but not back in to it.  When you first power up the system the highest voltage pack will supply the bulk of the current until it's voltage falls a bit at which point the others will start to supply current as well.  I used 3 x 20TQ045 20A 45V diodes from RS Components (stock no 302-173) mounted on a terminal block attached to the inside wall of the box.  I also fitted an isolating switch which disconnects the battery packs from the rest of the electronics as a belt and braces safety measure.

NiMH cells do not like to be deeply discharged, especially when they are connected in series.  I searched manufacturers' websites and most of them recommend not discharging below either 1.1V or 1.0V per cell.  I fitted a small voltmeter to the switch panel on Beastbox so I could monitor the battery voltage in use and switch off at either 13.2 or 12V depending on which manufacturer I believed on the day.

Step 8: Amplifiers

Most likely you will be using car audio amplifiers for your system.  Most of these are not very efficient, converting a lot of your precious energy into heat.  The solution is to use Class D amplifiers if at all possible.  Certainly you really must use a Class D amp for the bass driver because that is where most of the power is being used.

Beastbox uses a Kenwood KAC-6104D single channel amp for the bass driver and a JVC KS-AR8002D stereo amp with one channel driving the midrange and the other the tweeter.  If your bass amp has a subsonic filter make sure this is enabled because a 6th order bandpass box does not provide much loading outside of it's pass band and you can damage the bass driver if you throw low frequencies at it.

You can also buy modular, bare-board Class D amplifiers which would probably be a lot cheaper than using a car audio amp, but I only discovered this late on in the build. The ones I have looked at whilst writing this Instructable need a 40 - 60V power supply (often twin rail).  I don't know enough about electronics to design and build a suitable boost converter but if you have the know-how it would certainly be well worth looking at. 

Step 9: Crossover and Capacitor

To split the input signal into bass, midrange and high components you need a crossover. SoP uses a DIY crossover circuit and I did actually assemble the same circuit but I couldn't get it to work (I'm a mechanical engineer and I think I must have been using the wrong sized hammer...) so I gave up and used a commercial crossover unit instead.  A colleague at work is a car audio nut and he had several old active crossover units which I could try.  I finally decided on a Kicker KX3 crossover.  I mounted it in line with the front chamber port so that by stretching I can reach it from outside to change the balance when the system is running.

The same car audio nut had a very large (1.7F) capacitor like this one which he said he could "lend" me.  At first I thought I wouldn't need it and because it is quite heavy I was going to leave it out.  Then I thought about it and realised that if I could smooth out the current peaks drawn from the batteries they should last a lot longer (internal losses in the cells go up with the square of the current) so I decided to install it.  It is connected to the batteries just after the isolating switch by a 100R resistor so that when you turn on the isolator the capacitor charges slowly through the resistor.  Once the capacitor is charged I have a second switch which connects it directly across the batteries.  I don't know how much difference it makes as I have never tried running the system with it disconnected but I measured the current draw from the cells and it certainly seems to be doing something!  It also has a voltmeter so you can use it to monitor the battery voltage but to see it you have to peer in to one of the bass ports which is not very convenient (and not very good for your ears).

Step 10: Amplifiers Update Dec 2012

The main problem I've had with Beastbox is that it is very heavy. The assembled system with batteries in place on its trailer weighs about 56 kg. When you're towing it on the flat that's not too much of a problem but as soon as you come to any sort of incline it suddenly starts to get very hard work. Of course the heaviest components are the bass driver, the enclosure itself and the batteries but the two amplifiers and the capacitor weigh about 7 kg between them.

I mentioned earlier that it is possible to buy high power class D amplifier modules very cheaply from websites like eBay. The problem with these modules is that they usually require 45 to 50 V DC to operate and the battery packs in Beastbox only deliver 14.4 V. However, the three battery packs are connected in parallel and rather stupidly I hadn't thought of the option of connecting them in series! Three 14.4 V packs in series gives a useful 43.2 V and in fact when the packs are fully charged their terminal voltage is nearer 15 or 16 V so when they are in series you can get almost 50 V out of them. Once I had realised this I did some searching on eBay and found this amplifier module. It is based on the TAS5630 amplifier chip and delivers 300 W per channel into a 4 Ohm load and runs off 40 to 50 V. The best part is that it only costs £35.42 which is less than half the cost of one of the amplifiers that I was currently using!  Plus it is tiny!  And light!

I knocked up a harness to connect the three battery packs in series and tested the amplifier on the bench.  It sounded pretty good so I stripped out the two existing amplifiers, the capacitor and quite a lot of spare wiring and replaced it all with the new amplifier module.  The amplifier is connected to the crossover so that one channel feeds the bass driver and the other channel feeds the new midrange driver (see earlier update). The third crossover channel is redundant now.

The amplifier has three gain settings, low, medium and high. I experimented with all three settings whilst measuring the current coming from the battery packs and decided that the medium setting was probably the one that I needed to use. Bear in mind that it's quite difficult to test Beastbox at full volume indoors because it is so loud. I tried it for the first time on the November London Critical Mass and at the end of the evening the battery packs were only about one third discharged so I have since changed the gain to high.

This change has considerably simplified the layout and wiring and also made the main unit about 6 kg lighter. It also opens up the possibility of using different types of batteries. For example 4 x 12 V sealed lead acid batteries would give 48 V if you wired them in series. Alternatively, a four cell lithium polymer battery gives 14.8 V (and probably a bit more than that under no load when fully charged) so three of them in series could also power the amplifier quite successfully.

The only disadvantage of this new setup is that the crossover still needs 12 to 14 V to operate. A while ago I had also fitted a Bluetooth module so that I could connect my iPod wirelessly and this module also needs 12 V to run. I could have tapped off one of the main battery packs to power the accessories but I decided to fit a separate battery just to power these items. I fitted a 5 amp hour 4 cell lithium polymer battery with an in-line 5A fuse and used two of the existing switches on the control panel so that I could switch on the Bluetooth and the crossover independently.  This also means I can use the big isolator switch just for the main battery power.

Step 11: Wiring

The wiring looks complicated in the photos but it's pretty straightforward.  I used the space above the larger port tube to house a small control panel which has room for a 4 way connector, 3 switches and the voltmeter.  The batteries are connected via the diodes to the main isolating switch.  The capacitor is connected across the batteries in series with a 100R resistor and also by a 20A relay.  The relay coil is powered via the first switch on the control panel.  The relay contacts are wired so that when closed they connect the capacitor directly across the battery output, bypassing the 100R resistor.  I found I had to wire the capacitor this way because otherwise it would pop the circuit breakers on the battery packs when connected.

The second and third switches connect the positive feed from the batteries to the remote turn on lines to the crossover and the amplifiers.  This allows me to power up the system in sequence
  • turn on the isolator
  • wait until the capacitor has charged
  • switch the capacitor to directly connected to the batteries
  • turn on the crossover
  • finally, turn on the amplifiers
Having a separate switch for the amplifiers also allows me to turn just the amplifiers off if I want to change sound source.

The little switch turns the voltmeter on and off.  It has to run off 9V and for simplicity I just mounted a 9V battery under the panel!  The big 4 pole connector is for the feed to the midrange and tweeter so that they can be removed for transport or storage.  The connector is supposedly waterproof but I haven't put it to the test yet.  We shall see what happens the first time I have the system out in the rain!

The audio input is via two RCA connectors on the outside of the enclosure.  Although the system is mono and would therefore only need a single input, I wasn't sure how the crossover would react to only having one channel driven so I fitted two input jacks.  I set my iPod to mono output so the two channels are the same anyway.  The crossover has three outputs - woofer, front and rear.  The crossover frequencies between the outputs are adjustable over a reasonable range.  The woofer output goes to the bass amp which is set to single channel mode.  The front and rear outputs feed the left and right channels of the JVC amp, the outputs of which go to the midrange horn and the tweeter.

I carry my iPod in my jacket pocket and have a long curly lead which is tie-wrapped to my bike frame to connect me to BeastBox.  A better solution might be some sort of short range radio link (Bluetooth?) but I haven't investigated this yet.

Step 12: Finishing Touches

I painted the inside of the main enclosure with white gloss paint and the outside in a rather sickly purple colour.  In keeping with the Transformers theme I got a nice yellow laser cut vinyl Decepticons logo to go on the rear end.  I added a couple of FibreFlare lights down each corner for safety.  At some point I would like to fit some LED lights inside the ports and maybe underneath for that full custom car look.

To mount the system on the trailer I use three ratchet tie-down straps, one around the front end of the box holding the cover in place, one around the rear end and one over the top of the the midrange assembly which also serves to hold the cover down.  The cover needs to be clamped firmly in place as the bass driver generates significant overpressure and will easily lift the cover off if it is not secure.  Also bear in mind that your trailer is unlikely to have any suspension and you don't want 50+kgs of woodwork and electronics falling off the trailer in the middle of the road!

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